Liquid crystal display device

ABSTRACT

A liquid crystal display device of the active matrix kind having an array of picture elements (10) defined by opposing electrodes (34;45,14), carried on respective supporting plates (20,22) with switching elements (16), such as two terminal bidirectional diode structures, arranged serially between address lines (12) and one set of picture element, PE, electrodes on one plate, in which the one set of PE electrodes overlie the address lines with the switching elements disposed therebetween, thus allowing close packing of the elements. For transmissive mode operation the PE electrodes and address lines, and maybe the switching elements, are transparent. A high degree of fault tolerance is obtained by providing a plurality of switching elements for each picture element, and dividing each of the one set of PE electrodes into a plurality of sub-electrodes, each associated with a respective one or more switching elements. The switching elements may be capacitively coupled to the PE electrodes. The device may further include conductive layers on the one plate forming storage capacitors associated with the picture elements and serving to screen the PE electrodes.

This invention relates to a liquid crystal display device comprising apair of spaced supporting plates, liquid crystal material between theplates, a plurality of picture elements in a matrix array, each pictureelement being defined by opposing electrodes provided on the supportingplates, and a plurality of switching elements carried on one of thesupporting plates and connected in series between address lines on thatplate and the picture elements.

A liquid crystal display device of this kind, which is more preciselyreferred to as an active matrix addressed liquid crystal display device,is suitable for displaying alpha-numeric or video information.

In one known form of such a display device, thin film transistors, TFTs,are used as the switching elements. The picture elements, and likewisethe TFTs, are arranged in rows and columns and sets of column and rowaddress lines are carried on the one supporting plate. The gates of allTFTs in each row are connected to a respective row address line and thesources of all TFTs in each column are connected to a respective columnaddress line. The drains of the TFTs are connected to individual pictureelement electrodes provided on that plate. A line-at-a-time addressingtechnique is usually employed with each row of TFTs being switched "on"in turn and, for example, video information signals simultaneouslyapplied to the column address lines, these signals appearing on thepicture element electrodes via the "on" TFTs in the rows. After a setperiod, the row of TFTs is turned "off" and the next row of TFTs turned"on". The video information across the picture elements in the firstaddressed row is maintained, due to the natural capacitance of thepicture element and the high impedance of the TFTs in their "off" state,until the next time that row is addressed, that is, during the nextvideo field.

In other forms of display devices a simplified structure is achievedusing two terminal non-linear elements as the switching elements. Theseelements may be in the form of diodes structures for example asdescribed in U.S. Pat. Specification No. 4,223,308, or as described inBritish Patent Specification No. 2,091,468 in which the diode structurescomprise MIMs (Metal-Insulator-Metal structures). In these devices, thepicture elements are arranged in rows and columns with one of thesupporting plates carrying, for example, rows of address lines and otherplate carrying columns of address lines each of which is connected viathe series two terminal switching elements to the individual pictureelement electrodes associated with that column.

In all the above-described known forms of liquid crystal displaydevices, it is common for the switching elements and the associatedaddress lines to be positioned laterally of their associated pictureelement electrodes on the one supporting plate and connected with thepicture element electrodes by means of extensions formed integrally withthe electrodes. Accordingly, when considering the available area of thedisplay device, a certain proportion of the available area is devoted tothe switching elements and associated address lines, and the pictureelement electrodes are restricted to the remaining available area. Inorder therefore to maximise the proportion of available area occupied bythe actual picture elements, it has been necessary to keep the physicalsize of both the switching elements and the associated address lines toa minimum.

It has long been recognised that the nature of the active matrixaddressed display devices renders them difficult to manufacturesufficiently reliably in large area arrays and problems with yield havebeen common. Failure of the individual switching elements causes pictureelement defects and can lead to whole rows or columns of pictureelements being defective rendering the device unusable. In an attempt toalleviate this problem of yield, it has been proposed in TFT typedisplay devices that each picture element be associated with more thanone, typically two, TFTs for redundancy purposes. However, the provisionof additional switching elements results in a further reduction in thearea available for the actual picture elements and, for a given size ofdisplay device, therefore increases the ratio of non-effective displayproduction areas to effective display production area.

It is an object of the present invention to provide an active matrixaddressed liquid crystal display device in which the ratio ofnon-effective to effective display production areas can be minimised.

It is another object of the present invention to provide an activematrix addressed liquid crystal display device which has a degree offault tolerance so that it is less susceptible to the effects ofaddressing element defects.

It is a further object of the present invention to provide an activematrix addressed liquid crystal display device which lends itself tocomparatively simple manufacturing processes and which is economic tomanufacture.

According to the present invention there is provided a liquid crystaldisplay device as described in the opening paragraph which ischaracterised in that the picture element electrodes on the onesupporting plate overlie both the switching elements and the associatedaddress lines on that one supporting plate with the switching elementsbeing disposed between those picture element electrodes and addresslines.

With such a display device, the need to reserve areas on the onesupporting plate to accommodate the provision of laterally arrangedswitching elements and addressed lines, as in the aforementioned earlierconstructions, is avoided. By using a vertical, rather than lateral,structure the picture elements can be disposed closer together. Hence,the area of the one supporting plate which can be devoted to the pictureelement electrodes can be increased, leading to a maximisation of theratio of the actual area effective in producing a display to theremaining area not contributing to the production of a display, and amore effective utilisation of the supporting plate area for displayproduction purposes.

Moreover, with such a vertical structure, each of these address lines onthe one supporting plate can have a width up to the width of the pictureelement. In practice, the transverse dimension of the address lines maytherefore be substantially the same as that of the picture element withthe transverse dimension of the address lines serving to define thecorresponding dimension of the picture elements.

The display device may be adapted for operation in the reflective mode.In a preferred embodiment, however, the device is adapted to operate inthe transmissive mode. In this case, and in order that the address lineson the one supporting plate do not impede the passage of light throughthe picture elements significantly, those address lines are madesubstantially transparent, for example, of indium tin oxide (ITO).

For simplicity and convenience the switching elements are preferably twoterminal, bidirectional, non-linear devices such as diode structures. Inview of the fact that such diode structures, for example MIMs(Metal-Insulator-Metal structures) can readily be formed in a very smallsize relative to the picture element, their presence below the pictureelement need not affect significantly the passage of light through thedisplay device. In order to avoid any possibility of light transmissionbeing impeded and to achieve optimum transmission properties, theswitching elements may be formed so as to exhibit substantialtransparency to light. For example, the elements may comprise asubstantially transparent diode structure comprising a thin film ofsilicon nitride sandwiched between two layers of ITO and acting in themanner of a MIM structure with the ITO layers serving in effect as themetal components.

In view of their transparency, it is not necessary to restrict thephysical dimensions of the switching element to minimise the area theyoccupy, as is the case with, for example, laterally-arranged TFTs, tomaximise the utilisable display area.

In a preferred embodiment of the invention, a plurality of two terminalswitching elements are provided for each of the picture elements forfault tolerance purposes, the elements being operable in parallel withone another with their respective first terminals connected to theassociated address line. In the case where the display device isintended to be operated in the transmissive mode, the switching elementscan be formed as substantially transparent elements, so that theprovision of pluralities of switching elements beneath each of thepicture element electrodes does not impair light transmission throughthe picture elements, although this may not be necessary in view of thesmall size of the individual switching elements relative to the size ofthe picture elements. The number of transparent switching elements whichcan be provided for each picture element is not limited by lighttransmission considerations but merely by the relative physicaldimensions of the switching elements and the picture elements.

In order to utilize the provision of a plurality of switching elementsfor each picture element most beneficially for fault tolerance purposes,each picture element electrode on the one supporting plate preferablycomprises a plurality of discrete sub-electrodes each of which isconnected to a respective one or more of the plurality of switchingelements. In this manner, each picture element is, in effect, dividedinto a number of sub-picture elements each being individuallycontrollable through its associated one or more switching elements.Thus, when a display signal is applied to the picture elements, theindividual sub-picture elements of each of the picture elements areswitched independently of one another via their respective switchingelements to produce a display effect from the picture element. Anacceptable display effect can still be achieved even if one or more ofthe sub-picture elements is non-operable, depending on the number ofsub-picture elements and their size relative to the picture elementitself. Although the display effect produced by a picture element havinga few defective sub-picture elements will inevitably be affected to someextent, this may not be perceived by a viewer when viewing a devicecomprising a great number of picture elements typically hundreds ofthousands, each of which occupies an area of only, say, 300 by 300micrometers. The device may therefore still be used with satisfactoryresults with a number of picture elements being affected in this mannerunlike known devices employing a single switching element for each wholepicture element where failure of a switching element results in completefailure of at least the associated picture element, and perhaps rendersthe device unusable.

As in conventional matrix liquid crystal display devices, the pictureelement electrodes on the one supporting plate are generally planar,extending in a plane substantially parallel to the facing surface of thesupporting plate, and in this case, therefore, the plurality ofswitching elements associated with the picture element conveniently maybe arranged substantially in a planar array underlying the pictureelement electrodes. The array of switching elements associated with eachpicture element may occupy an area corresponding with at least a majorportion of the area of the picture element electrodes and preferably thearray is such that the switching elements of the array are arranged, andspaced from one another, substantially uniformly with respect to thearea of the picture element electrodes. In other words, the plurality ofswitching elements for each picture element can be spread out to occupyindividual and evenly-spaced discrete regions of an area beneath thepicture element electrode which corresponds substantially to the area ofthe picture element.

The use of diode structures as two-terminal, bidirectional, switchingelements is especially attractive and enables further advantages to beobtained. As previously mentioned, the diode structures may be a kind ofMIM comprising indium tin oxide (ITO) layers as the "metal" componentsand silicon nitride, for example in the form of non-stoichiometricsilicon nitride, as the insulator component sandwiched therebetween,these materials being substantially transparent to visible light in thinlayer form. Alternatively, silicon oxide may be used instead of siliconnitride. Another form of MIM type element which could be employed maycomprise an anodised tantalum film with an overlying conductive layer,which need not necessarily be transparent. In one embodiment of theinvention, a deposited conductive layer, for example of ITO, and actingas one of the conductive terminal layers of the diode structure may alsoserve as the picture element electrode. In other words, the pictureelement electrode is formed integrally with one of the diode structure'sconductive terminal components as a unitary layer. The other conductiveterminal component of the diode structure may conveniently beconstituted by a respective portion of one of the set of paralleladdress lines carried on the one supporting plate, the first-mentionedconductive terminal layer being spaced vertically of the supportingplate from this address line by a thin layer of insulator, for example,silicon nitride. The other supporting plate of the display devicecarries a second set of parallel conductors arranged at right angles tothe first set, in an x-y configuration, with areas of overlap betweenthe first and second sets of conductors defining the picture elementregions.

In another embodiment, similarly employing crossed sets of conductors onthe supporting plates, and MIM-like diode structures, the conductiveterminal layer of the diode structures remote from the one supportingplate and facing the liquid crystal material is formed separately fromthe actual picture element electrodes, these comprising a superimposedconductor layer, for example of ITO, and are spaced therefrom byinsulator material such as silicon nitride so as to form a capacitivecoupling. As before, the other conductive terminal layer of the diodestructures is constituted by a respective portion of one of the set ofaddress lines carried on the one supporting plate.

In this embodiment, the first-mentioned conductive terminal of the diodestructures may be constituted by grains of conductive material, forexample, ITO, dispersed in an insulator matrix material and spaced fromthe other conductive terminal of the diode structure by a thin film ofinsulator. This dispersion of one of the metal components of the diodestructures may be random but with sufficient density to ensure that eachpicture element sub-electrode is associated with, through capacitivecoupling, at least one so-formed diode structure , and preferably aplurality of diode structures.

This manner of diode structure formation avoids the use of conventionalphotolithographic procedures and can be accomplished using simplertechniques, for example by using powdered ITO and sprinkling the ITOgrains over the thin insulatr layer carried on the address lines on theone supporting plate.

In a variant of this embodiment the metal component of the diodestructures remote from the one supporting plate are formed as discretedots of conductive material, for example, ITO, deposited via a mask sothat the positioning and spacing of the diode structures so-formed areto a considerable extent predetermined rather than random. Thisdeposition may involve evaporation of the conductive material through asuitable mask which in a convenient form may comprise a structuresimilar to a shadow mask. In addition to avoiding the need for aphotolithographic operation, this technique has the further advantagesthat it is comparatively inexpensive and, in comparison with theaforementioned random dispersion technique, the number of diodestructures provided for each picture element, and more precisely thenumber of diode structures, whether it be one or more, associated witheach picture element sub-electrode, can be controlled.

In any of the aforementioned embodiments or variants, each pictureelement of the liquid crystal display device may have associatedtherewith at least one storage capacitor. In the case where each pictureelement electrode on the one supporting plate comprises a plurality ofdiscrete sub-electrodes each serving to define a respective picturesub-element, then each picture sub-element preferably has associatedtherewith a respective storage capacitor. These storage capacitors maybe constituted in part by at least one further conductive layer carriedon the one supporting plate which underlies and is insulatedelectrically from the picture element electrodes on the one supportingplate. Besides constituting in part the storage capacitors, thesefurther conductive layers have another useful and advantageous functionin that they may be situated between the picture element electrodes andthe underlying address lines and serve to screen the picture elementelectrodes from their underlying address lines. As a result of thisscreening, any capacitance between a picture element and its addressline is minimised.

Various embodiments of liquid crystal display devices accordance withthe present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows, in plan schematic form, a part of an active matrix liquidcrystal display device according to the invention having a matrix arrayof picture elements controlled by two terminal non-linear switchingdevices;

FIG. 2 is a diagrammatic cross-sectional view, not to scale, of oneembodiment of display device according to the invention;

FIG. 3 is a scrap, diagrammatic plan view of part of the display deviceof FIG. 2;

FIG. 4 illustrates schematically the effective electrical circuit of atypical one of the picture elements of the device of FIG. 2;

FIG. 5 is a diagrammatic cross-sectional view, not to scale, of anotherembodiment of a display device according to the invention;

FIG. 6 is a diagrammatic cross-sectional view, not to scale, of afurther embodiment of a display device according to the invention; and

FIG. 7 is a schematic illustration of the effective electrical circuitof a typical one of the picture elements of the devices of FIGS. 5 and6.

FIGS. 8 and 10 illustrate variants of the display devices shown in FIGS.2 and 5 respectively, again not to scale, in which storage capacitorsare provided for the picture elements;

FIGS. 9 and 11 are schematic effective circuit representations, similarin respects to the circuits shown in

FIGS. 4 and 7, of typical ones of the picture elements of the devices ofFIGS. 8 and 10 respectively; and

FIG. 12 shows a diagrammatic cross-sectional view, not to scale, throughanother display device according to the invention also incorporatingstorage capacitors for the picture elements, and in which the effectivecircuit of a typical picture element is generally similar to that shownin FIG. 11.

Referring to FIG. 1, which is applicable to all the various embodimentsand variants described hereinafter, there is shown a part of an activematrix liquid crystal display device having a large number of pictureelements, 10, arranged in a matrix array of rows and columns andcomprising spaced electrodes with liquid crystal material therebetween.Only six picture elements are illustrated in FIG. 1. The pictureelements 10 are addressed via two sets of address lines in the form ofconductors 12 and 14 carried on the facing surfaces of two glasssupporting plates (not shown in FIG. 1) on which the picture elementelectrodes are also carried. The conductors of each set are arrangedparallel to one another and the two sets extend at right angles to eachother in their spaced-apart planes and define at their intersectionpoints the picture element regions.

The row conductors 12 serve as scanning electrodes and are driven by adriver circuit (not shown) which applies a scanning signal to each rowelectrode 12 sequentially in turn. In synchronism with these scanningsignals, data signals are applied to the columns of conductors 14 toproduce the required display from the rows of picture elementsassociated with the row conductors 12 as they are scanned. In the caseof a video or TV display device, these data signals comprise videoinformation signals. By appropriately choosing the difference betweenthe scanning and data signals to be sufficiently great, the opticaltransmissivity of a selected picture element at the intersection of arow conductor 12 and a column conductor 14 can be changed to produce avisible display effect. The picture elements are only activated toproduce a display effect in response to both the scanning signal anddata signal being applied thereto by means of switching elements 16 inthe form of non-linear elements connected in series between each pictureelement 10 and its row conductor 12. The scanning signal applied to arow conductor 12 causes the switching elements associated with thepicture elements of that row to conduct and thereby pass the scanningsignal to the picture element. The scanning signal in conjunction withthe data signal produces the desired optical effect from the pictureelement. The data signal by itself is insufficient to cause such aneffect. The individual display effects of the large number of pictureelements, addressed one row at a time, combine to build up a completedisplay. Using the transmission/voltage characteristics of a liquidcrystal picture element, grey scale levels can be achieved.

The voltage/conduction characteristic of the switching element ideallyis bidirectional and symmetrical with respect to zero voltage so that anet dc bias across the picture elements, and consequentialelectro-chemical degradation of the liquid crystal material, is avoided.However, a voltage/conduction characteristic close to the ideal isacceptable. For convenience the polarity of the drive voltages, that is,the scanning and data signals, is reversed after each complete field.

Active matrix liquid crystal display devices employing two terminal,non-linear switching elements in series with, and laterally spaced withrespect to, the picture elements are generally well known. The mainelements and general operation of the display device of FIG. 1 issimilar in certain respects these known devices and accordingly theabove description with regard to FIG. 1 has deliberately been keptbrief. For further information on these aspects, therefore, reference isinvited to earlier publications describing these generally similar typesof display devices such as, for example, U.S. Pat. No. 4,223,308 andBritish Patent Specification No. 2,147,135, both describing the use ofdiodes as switching elements, and British Patent Specification No.2,091,468, describing the use of MIMs as switching elements, details ofwhich we incorporated herein by reference. Other forms of non-linearswitching elements may instead be used in the present invention, forexample p⁺ -i-p⁺, n⁺ -p-n⁺ diode structures.

In the following description, various embodiments of display devices andvariants in accordance with the invention will be described. In each ofthese embodiments, the picture element electrodes carried on one of thesupporting plates are, for fault tolerance purposes, each associatedwith a plurality of separately-operating switching elements which aredisposed between those picture element electrodes and the associatedaddress line disposed on the supporting plate and underlying the areaoccupied by the picture element electrodes. Furthermore, each pictureelement electrode is divided into a number of discrete sub-electrodeswhich are each connected to at least a respective one of the pluralityof switching elements associated with that picture element so thatvoltages applied to those sub-electrodes are controlled by therespective switching elements. Although the liquid crystal displaydevice according to the invention may be operated in reflection mode,the following embodiments are all intended to be operated intransmission mode. With this in mind, the address lines on theaforementioned supporting plate are formed substantially transparent soas not to impede the passage of light through the picture elementssignificantly. While not essential in view of their size physicallyrelative to the picture element size, the switching elements are alsosubstantially transparent and, in these particular embodiments, comprisediode structures operating as MIM thin-film structures utilising thePoole-Frenkel effect. As examples of typical dimensions, the pictureelements may be approximately 300 by 300 micrometers square, and wheneach picture element has, say, nine sub-electrodes associatedrespectively with a single switching element, each sub-electrode may beapproximately 100 by 100 micrometers and each switching elementapproximately 5 to 10 micrometers square.

For simplicity, the same reference numerals are used to designatecorresponding parts of the various embodiments.

Referring to FIG. 2, there is shown in schematic form a cross-sectionalview through a first embodiment of active matrix display deviceaccording to the invention. The device comprises two mutually spacedtransparent supporting plates 20 and 22 whose facing surfaces at leastare insulative and between which liquid crystal material 24 is disposed.The plates may be of any suitable material, for example, glass.

The upper supporting plate 20 carries the set of column conductors 14,one of which is visible in FIG. 2, formed of transparent conductivematerial such as indium tin oxide (ITO). The conductors 14 andintervening areas of the plate surface are covered by an insulativeorientation layer 25 of polyimide.

The lower supporting plate 22 carries the set of row conductors 12, twoof which are shown in FIG. 2, which, again, are formed of transparentconductive material such as ITO. The two sets of conductors 12 and 14,as previously mentioned, extend substantially at right angles to oneanother and define at their intersections the locations of the matrix ofpicture elements, here designated 10. The conductors 12 and 14 as shownare each formed as flat-surfaced strips of substantially constant width,producing substantially rectangular picture elements. Alternatively, theconductors 12 and 14 need not be formed as unitary strips but maycomprise discrete areas of conductive material at the picture elementsregions interconnected by separately-formed conductive tracks.

Disposed over the conductors 12 and the intervening surface areas of theplate 22 is an insulative layer 27, deposited either as a single layeror a plurality of separate layers, of silicon nitride, although silicondioxide may be used instead. Portions of this silicon nitride layer 27at the regions where the conductors 12 and 14 intersect, andcorresponding in area substantially with the individual picture elementareas determined by the area of overlap between the conductors 12 and 14at their intersections, are formed with pits 30 by selective etchingphotolithographically through a mask. Each such portion of the layer 27has a matrix of regularly spaced pits, this being a 4 by 4 matrix in theexample illustrated in FIG. 2 although only four such pits are visiblein the cross-section shown, which occupy an area correspondingsubstantially with the picture element area over the conductors 12 andare spaced substantially uniformly with respect to that area.

Following etching of these pits 30 a thin film, around 150 Angstroms, ofnon-stoichiometric silicon nitride is deposited over the structure thusforming a thin film of insulator on the surface of conductors 12 at 32the bottom of each pit.

Discrete dots 34 of transparent conductive material, such as ITO, aredeposited over the layer 27 and extend into respective ones of thesepits, either by deposition through a mask or by selective etching of acontinuous layer deposited over the layer 27. Each dot 34 is in the formof a rectangular or circular layer which extends over its respective pit30 and laterally over the immediately adjacent surface of the layer 27surrounding the pit, as shown in FIG. 2. Thus, each picture element 10has a 4 by 4 regular matrix of dots 34 (sixteen altogether), which, whenviewed from above in FIG. 2, and as shown in FIG. 3, occupy a majorproportion of the area of the picture element and are spaced apart fromone another uniformly within that area. These dots 34 constitute thepicture element electrodes associated with the supporting plate 22, eachsuch picture element electrode in effect being divided into sixteenindividually-energisable sub-electrodes 34, defining a correspondingnumber of picture sub-elements, C_(LC), which collectively serve as thepicture element electrode on the supporting plate 22.

Each 4 by 4 matrix of sub-electrodes together with the overlying portionof a conductor 14, acting as a spaced electrode, and the liquid crystalmaterial therebetween constitute a picture element.

The thin layer of silicon nitride 32 at the bottom of each pit 30together with respective immediately adjacent surface portions of theconductors 12 and dots 34 constitute a two terminal, non-linear diodestructure which is connected electrically in series between theconductor 12 and the associated picture element sub-electrode defined bythe dot 34. The sixteen sub-electrodes of each picture element aretherefore connected to the same conductor 12 through a respective diodestructure.

A further polyimide orientation layer 35 is deposited over the layer 27and dots 34.

The effective electrical circuit configuration of a typical pictureelement and associated switching elements of the display device of FIG.2 is illustrated in FIG. 4. For simplicity, only three of the sixteensub-electrodes and associated diode structures are shown, and anyparasitic capacitances which might be present have been ignored.

The MIM type diode structures, being two terminal bidirectionalnon-linear devices, are here represented, at 38, by back-to-back diodesfor convenience. The application of a suitable voltage comprising thescanning signal and data signal to he electrodes 12 and 14 respectivelyexceeding the threshold voltage of the diode structures to the rowconductor 12 causes the diode structures 38 to "switch" and conductunder the Poole Frenkel effect so that the voltage is transferred to thedot sub-electrodes 34. This voltage, in conjunction with a furthervoltage signal (the data signal) applied simultaneously to the relevantconductor 14 produces in each of the picture sub-elements, defined bythe sub-electrodes 34, the required electro-optic effect. Below thisthreshold voltage, the diode structures 38 are non-conductive. Becausethe diode structures are bidirectional and substantially symmetricdevices, the polarity of the applied voltages can be reversed insuccessive fields so as to avoid a DC component acting on the liquidcrystal material and the consequential electro-chemical degradation ofthe material.

Such a device is relatively simple, and inexpensive, to construct.Moreover, in the event of failure of one, or even several, of thepicture sub-elements through defects in the associated diode structures,the picture element is still capable of functioning adequately toproduce an acceptable display effect. Since all the component layers aresubstantially transparent, the passage of light through the device whenoperated in transmission mode is not significantly impeded by thepresence of the plurality of switching elements beneath the pictureelement electrodes. The size of the diode structures 38 shown in FIG. 2have deliberately been exaggerated and in reality the area they occupyrelative to the picture element area is small. Hence, even if the diodestructures 38 are not transparent, their effect on light transmissionneed not be very significant.

Referring now to FIG. 5, there is shown in schematic form across-section through a second embodiment of active matrix liquidcrystal display device according to the invention. This embodimentshares many similarities with the previous embodiment. For this reasoncorresponding component parts are designated with the same referencenumerals and will not be described again here in detail.

An important distinction between the embodiment of FIG. 5 and that ofFIG. 2 is that the MIM type diode structures are formed in a differentmanner and their terminals remote from the address conductors 12 are notconnected directly to the picture element sub-electrodes but instead arecapacitively coupled to those sub-electrodes.

A thin layer 40 of non-stoichiometric silicon nitride is deposited to athickness of around 0.05 micrometers (500 Angstroms) over the conductors12 and the intervening surface area of the supporting plate 22. Siliconoxide may be used instead of silicon nitride. Over the surface of thislayer 40 is disposed a further, thicker, layer 41 of insulative materialsuch as silicon nitride or polyimide in which ITO grains 43 restingdirectly on the thin layer 40 are embedded. These grains 43, which aregenerally spherical and substantially similar in size, say, around 1micrometer diameter, are dispersed randomly in the layer 41 in densitysufficient to ensure that a suitable number of grains, for examplearound 15 to 100, are present scattered over the surface of the layer 40on the conductors 12 at each picture element location.

The grains 43 may be dispersed over the layer 40 as a suspension inalcohol. Following evaporation of the alcohol, the layer 41 is depositedusing silicon nitride so as to cover and surround the grains apart fromthe region where they contact the thin silicon nitride layer 40. In analternative method, in which the layer 41 comprises polyimide, thegrains 43 may be mixed thoroughly with polyimide and the mixture thenspun over the surface of the layer 40, the grains then being allowed tosettle on, and contact the thin layer 40. Both these depositiontechniques offer the advantage that they avoid the need for aphotolithographic process and are comparatively simple, and economic, toperform.

While the dispersion of the grains 43 in the layer 41 is accomplished ina relatively random fashion, the quantity of grains used in proportionin the material of the layer 41 results in sufficient numbers of grainsbeing present at each picture element location for the desired purpose,as will be explained. In the example illustrated in FIG. 5, theresulting distribution of grains 43 is such that several grains areobtained, in this particular case five and six, across the width of eachcomplete conductor 12 which are spaced substantially uniformly. Ofcourse, the grains are not necessarily aligned with one another exactlyas shown in this schematic representation. The grains are similarlydistributed along the lengths of the conductors 12, and hence along theother dimension of the picture element. If an area of the conductors 12corresponding to a typical picture element area is considered, aroundtwenty-five grains 43 arranged in a planar two dimensional array will beinvolved. Whilst the spacing between grains can not be determinedprecisely, their distribution will usually be such that, in each ofthese areas, the resultant array occupies a major proportion of thearea.

Each of the grains 43 which overlie a conductor 12 in conjunction with aportion of the thin silicon nitride layer 40 and surface portion of theelectrode 12 immediately beneath it constitute a MIM type diodestructure.

The layer 41 has a thickness greater than the size of the grains 43 sothat the uppermost surface of the grains are spaced from the surface ofthe layer 41. On the surface of this layer 41 are deposited discreteelectrode layers 45 of, for example, ITO, which constitute pictureelement sub-electrodes. Each picture element for example has associatedtherewith four such sub-electrodes arranged in a planar 2 by 2 matrixarray, although the actual number of sub-electrodes 45 provided for eachpicture element may be varied if desired. The four sub-electrodes, whichare of similar size and may be rectangular or circular in shape,collectively constitute the picture element electrode on the supportingplate 22 and each defines with a corresponding overlying portion ofelectrode 14 and liquid crystal material therebetween a picturesub-element. Each sub-electrode 45 overlies a number, typically betweenfour to nine, diode structures which serve to control the application ofvoltages thereto. The sub-electrodes 45 are spaced from their associateddiode structures by silicon nitride material of the layer 41 so thatelectrical coupling between the diode structures and the sub-electrodesis achieved capacitively.

The sub-electrodes 45 and intervening surface areas of the layer 41 arecovered by a polyimide orientation layer 35.

The effective electrical circuit configuration of a typical one of thepicture elements of this embodiment is illustrated in FIG. 7. In thisfigure, the MIM type diode structures are represented by back-to-backdiodes 47 and the capacitive coupling between the diode structures 47and the sub-electrodes 45 by capacitors 48.

It will be appreciated that like the previous embodiment this embodimentoffers a high degree of fault tolerance. As each picture element 10 isassociated with a large number of diode structure switching elements,and each individual sub-electrode 45 itself is associated with aplurality of diode structures, defects in a number of the switchingelements can be tolerated without the operation of the picture elementsbeing seriously impaired. As with the previous embodiment, passage oflight is not significantly impeded when the device is operated intransmission mode by the presence of such large numbers of switchingelements beneath the picture element electrodes in view of the fact thatsubstantially transparent materials are employed to form these switchingelements. As before, the switching elements may be formed sufficientlysmall in relation to the picture elements that they have minimal effecton the transmission of light even if they are not substantiallytransparent.

This embodiment offers the further advantage over the previousembodiment in that variations in voltage applied across the electrodesof the picture elements arising from threshold variations in theindividual diode structures are reduced using capacitive divisionprovided by the plurality of capacitors 48 associated with eachsub-electrode 45. The capacitance value of the capacitors 48, which isdetermined by the size of the grains 43, is small relative to thecapacitance of the picture sub-elements, which is determined by the sizeof the sub-electrodes 45. Hence a comparatively large voltage dropexists across each capacitor 48. If therefore, for example, theeffective capacitance of the plurality of parallel-connected capacitors48 associated with one picture sub-element is one tenth of thecapacitance value of the picture sub-element, then a one volt variationacross the diode structures 47 associated with that sub-element givesrise to only 100mv variation across the sub-element.

Operation of the display device is otherwise generally similar to thatdescribed with regard to the embodiment of FIG. 2. It is necessary toreverse the polarity of the applied drive voltages after each field to"reset" the picture elements otherwise charge will be fixed in eachpicture element. Such polarity reversal serves also to preventelectrochemical degradation of the liquid crystal material as before.

Referring now to FIG. 6, there is shown, in schematic form, across-sectional view through a third embodiment of a display deviceaccording to the invention. This embodiment shows many similarities withthe embodiment of FIG. 5 and may be regarded as a variant of thatembodiment. Again, therefore, corresponding parts of the device havebeen designated the same reference numerals for simplicity.

In this embodiment, the grains 43 are replaced by discrete dots 50 ofconductive material such as ITO. The dots are deposited over the thinsilicon nitride layer 40 prior to deposition of the thicker siliconnitride layer 41 by evaporation through a mask so that the size andspacing of the dots is controlled to a considerable extent so that theyhave a regular spacing in relation to, and lie substantially inregistration with, the actual picture elements. The mask employed indepositing the dots 50 may be a shadow mask. The so-formed dots 50 havea substantially uniform distribution over the surface of the layer 40.The silicon nitride layer 41 is then laid down over the dots 50 andagain has a thickness greater than the height of the dots 50 so that theupper surfaces of the dots are spaced from the surface of the layer 41.The upper surfaces of the dots 50 and the opposing surface portionssub-electrodes 45 constitute, together with the silicon nitride of layer41 separating them acting as dielectric, capacitors coupling theassociated MIM type diode structures and sub-electrodes 45.

In this embodiment each picture element is provided with ninesub-electrodes 45 collectively constituting the picture elementelectrode and arranged in a planar 3 by 3 matrix array occupying an areasubstantially corresponding to the area of the picture element. The dots50 are so distributed that each sub-electrode is associated with threediode structures.

The electrical circuit configuration of a typical one of the pictureelements of this embodiment corresponds generally with that shown inFIG. 7, except that there are nine picture sub-elements rather than fouras shown, and each sub-element is associated with just three diodestructures 47 and three series capacitors 48. Operation of the device isgenerally the same as that of the previous embodiment with similaradvantages being obtained.

FIGS. 8 and 10 show respectively variants of the embodiments describedwith reference to FIGS. 2 and 5. As before, like parts have beendesignated with the same reference numerals. In each of these variantsthe display devices have been modified so as to minimise capacitivecoupling between the picture element sub-electrodes and the underlyingportions of their associated address lines and at the same time to forma storage capacitor for each picture sub-element.

The storage capacitors are designed to have capacitance values manytimes larger than the capacitance of the picture sub-elements.

With regard firstly to the variant of the embodiment of FIG. 2 shown inFIG. 8, it is seen that a further electrically-conductive layer 80 isincorporated in the device structure for each picture element. Theselayers 80, are transparent and formed, for example, of ITO. In view ofthe addition of the layers 80, the silicon nitride layer 27 comprisestwo separately-deposited layers 81 and 82, the second layer 82 beingdisposed over the first layer 81 and sandwiching the layers 80,deposited on the first layer prior to deposition of layer 82,therebetween. The layers 80 themselves are formed by depositing acontinuous ITO layer over the first silicon nitride layer 81 andselectively etching this layer to leave the required array of individuallayers 80 extending over an area substantially corresponding to, and inregistration with, their respective picture-elements 10. Each layer 80,and the layers 81 and 82 are formed with apertures through which theconductive layers constituting sub-electrodes 34 extend to contact thesilicon nitride thin films 32 with the layers 80 being spaced slightly,and hence electrically insulated from, their associated sub-electrodelayers 34, and also the underlying address lines 12.

The layers 80 of the picture elements 10 in each respective row areelectrically isolated from one another. The layers 80 of each pictureelement in each respective column are interconnected electrically, viaintegral bridges, not shown in FIG. 8, left following selective etchingof the continuous conductive layer constituting the layers 80, and eachinterconnected column of layers 80 is connected by means not visible inFIG. 8 with its associated column conductor 14.

The effective circuit configuration for a typical picture element of thedevice of FIG. 8 is illustrated schematically in FIG. 9. Comparing thecircuit of FIG. 9 with that of FIG. 4, it is seen that portions of thelayers 80 in conjunction with their respective overlying sub-electrodes34 and insulative layer 82 therebetween, constitute a capacitance C_(s)in parallel across each picture sub-element C_(LC). The capacitancesC_(s) serve as storage capacitors in operation of the device in knownmanner.

Referring now to FIG. 10, the display device illustrated is a variant ofthe embodiment of FIG. 5 with modifications similar to those describedwith reference to FIG. 8. In particular, additional transparent,electrically conductive, layers are interposed between the pictureelement sub-electrodes 45 and the underlying conductor 12. In this case,however, the additional conductive layer of each picture element isconfigured so as to define physically separated portions, referenced 90,each of which corresponds generally in size, and is registered with arespective picture element sub-electrode 45. However, the layer portions90 associated with the sub-electrodes 45 of each picture element arestill interconnected electrically, although these interconnections arenot visible in FIG. 10.

The layer portions 90, and their interconnections, are formed on thesurface of the silicon nitride layer 41 by the selective etching of adeposited continuous layer and are spaced from the sub-electrodes 45 bya further silicon nitride layer 91 extending continuously over thelayers 41 and 90. The layer portions 90, which correspond generally inshape with their associated picture element sub-electrode 45, areelectrically insulated from their associated overlying sub-electrode 45and underlying address lines 12. The layers 90 and 91 are provided withregistered openings through which the sub-electrodes 45 extend so as tocontact the upper surface of the layer 41 for capacitive coupling withthe diode structures underlying this extension in a manner similar tothat in the embodiment of FIG. 5. The extensions of the sub-electrodes45 are insulated from the layer portions 90 by material of the layer 91.Comparing the devices of FIGS. 10 and 5 therefore it will be seen thatin the configuration shown in FIG. 10, the picture elementsub-electrodes 45 are no longer wholly planar but have their peripheralportions surrounding the diode structure spaced further from theconductor 12 so as to accommodate the additional conductive layer 90 andthe necessary further insulation layer 91.

The layers 90 of adjacent picture elements in the same row areelectrically isolated from each other.

Again, the layers 90 associated with picture elements of each respectivecolumn of picture elements are interconnected electrically with oneanother (via columnwise bridges, not visible in FIG. 10, left followingetching of the continuous conductive layer) and with the respectivecolumn conductor 14.

The effective electrical circuit configuration of a typical pictureelement of the display device of FIG. 10 is illustrated in simple formin FIG. 11. Components corresponding to those in FIG. 7 are designatedwith the same reference numerals. As can be seen from this figure, eachpicture sub-element, C_(LC), has associated therewith a storagecapacitance C_(S) connected in parallel thereacross, this storagecapacitance being formed by the layer portion 90 and the opposingportion of sub-electrode 45, with the insulative material of layer 91therebetween.

Referring now to the embodiment shown in FIG. 12, this embodiment sharesmany similarities with that of FIG. 8 except that, as with theembodiment of FIG. 10, the picture element sub-electrodes, herereferenced again 34 for simplicity, are capacitively coupled to theirassociated switching elements. In this example, each picture element 10comprises a 2 by 2 array of picture sub-elements each of which iscapacatively coupled to one switching element. Components of the devicecorresponding generally with those of FIG. 8 have been designated thesame reference.

As before, a continuous conductive layer 80 is interposed between thesub-electrodes 34 of each picture element and its underlying addressline 12 and insulated therefrom by layers 82 and 81 respectively. Thelayer 80 is provided with apertures overlying pits 30 formed in thelayer 81. At the bottom of these pits, thin films of non-stoichiometricsilicon nitride 32 are deposited. However, instead of the sub-electrodes34 extending downwardly into the pits 30 to contact the thin films 32 aspreviously, they extend in this embodiment only part way into the pits.Deposited on the upper surface of the thin films 32 within the pits 30are respective conductive layers 95 formed simultaneously with, and thusof the same material as, the layers 80 prior to deposition of the layer82. The layer 82 is disposed over both the layers 80 and the layers 95in the pits 30 so that, when subsequently deposited, the conductivelayer constituting the sub-electrodes 34 extends downwardly over thelayer 82 in the region of the pits 30 and is spaced from the conductivelayers 95 by the layer 82.

The layers 95 together with their associated underlying films 32 andsurface portions of address lines 12 form MIM type diode structures asbefore, there being one diode structure for each sub-electrode, andhence each picture sub-element, of each picture element. These diodestructures are capacitively coupled to their respective sub-electrodes34.

Also as previously, the layers 80 of picture elements in a column areinterconnected electrically columnwise via bridges not shown in FIG. 12but are electrically isolated from the layers of adjacent pictureelements in the same row. Each interconnected column of layers 80 isconnected to the opposing column conductor 14.

The effective electrical configuration of a typical picture element issimilar to that shown in FIG. 9 representing a typical picture elementof the embodiment of FIG. 8 except that a coupling capacitance, similarto that illustrated at 48 in FIG. 11, exists in series between eachdiode structure switching element 38 and its associated picture elementsub-electrode 34. The layers 80 in conjunction with the overlyingsub-electrodes 34 and intervening insulation material of layer 82constitute a storage capacitor, Cs, for each picture sub-element.

With regard to the embodiments of FIGS. 8, 10 and 12, the conductivelayers 80 and 90 may be electrically connected to a source of fixedpotential rather than the associated column conductor 14, the polarityof this potential being reversed for alternate fields.

The embodiment described with reference to FIG. 7 can be modified in asimilar manner to that described with reference to FIG. 10 for theembodiment of FIG. 5 to incorporate storage capacitors by incorporatinga transparent conductive layer between each sub-electrode 45 and theunderlying address conductor 12.

In all these embodiments incorporating storage capacitors, the furtherconductive layers provided, 80 and 90, serve the additional usefulfunction of screening the picture element sub-electrodes 34 and 45 fromtheir underlying address lines 12 and so minimise any capacitivecoupling between the sub-electrodes and address lines which mightotherwise exist.

Although in FIGS. 8 and 12, the additional conductive layer 80associated with each picture element is shown as being continuous, theycould instead be defined as physically individual portions for eachsub-electrode, while still being electrically interconnected, in thesame manner as described with reference to FIG. 10. Conversely, theindividually defined layer portions 90 of the embodiment of FIG. 10could be formed as a continuous layer substantially co-extensive withthe respective picture element area but still provided with aperturesthrough which the sub-electrode layers 45 extend towards the switchingelements in a manner similar to that of FIGS. 8 and 12.

All the above-described embodiments and variants offer the advantageover known display device arrangements in that the spacing between thepicture elements can be minimised since the address lines on onesupporting plate and the associated switching elements are locatedbeneath the picture elements unlike the known arrangements in which theswitching elements are arranged on the supporting plate laterally withrespect to the picture elements.

I claim:
 1. A liquid crystal display device comprising(a) a pair ofspaced supporting plates, (b) a liquid crystal material disposed betweensaid supporting plates, (c) a plurality of picture elements disposed ina matrix array, wherein each of said plurality of picture elements isdefined by opposing electrodes disposed on said supporting plates, (d) aplurality of switching elements disposed on one of said two supportingplates, (e) a plurality of address lines disposed on said one of saidtwo supporting plates, wherein said picture element electrodes on saidone of said two supporting plates overlies both said switching elementsand associated address lines to said one of said two supporting plates,said switching elements being connected in series between said pictureelement electrodes and said address lines, and (f) at least one storagecapacitor disposed between each of said picture elements and said one ofsaid two supporting plates, wherein said storage capacitors include atleast one conductive layer disposed on said one supporting plate and anelectrically insulating layer disposed between said conductive layersand said picture element electrodes, said conductive layers screeningsaid picture element electrodes from associated underlying address linesto minimize capacitive coupling between said picture element electrodesand said associated address lines.
 2. A liquid crystal display deviceaccording to claim 1, wherein said switching elements are two terminal,bi-directional non-linear elements.
 3. A liquid crystal display deviceaccording to claim 2, wherein said switching elements include diodestructures.
 4. A liquid crystal display device according to claim 3,wherein said diode structures each include a thin layer of siliconnitride sandwiched between two electrically conductive layers of indiumtin oxide material, said electrically conductive layers formingrespective terminals of said diode structures.
 5. A liquid crystaldisplay device according to claim 2, wherein a conductive layer formingone terminal of said two terminal non-linear elements also serves as atleast part of an associated picture element electrode.
 6. A liquidcrystal display device according to claim 2, wherein insulators aredisposed between one terminal of said two terminal non-linear elementsand associated ones of said picture element electrodes to definecapacitive couplings between said switching elements and said pictureelement electrodes.
 7. A liquid crystal display device according toclaim 6, wherein conductive components forming said one terminal of saidtwo terminal non-linear elements include conductive grains dispersed inan insulator matrix material.
 8. A liquid crystal display deviceaccording to claim 6, wherein conductive components forming said oneterminal of said two terminal non-linear elements include discrete dotsof conductive material disposed through a mask.